Steel, Sheet Steel Product and Process for Producing a Sheet Steel Product

ABSTRACT

The invention relates to a steel and to a flat steel product produced therefrom that have optimized mechanical properties and at the same time can be produced at low cost, without having to rely for this on expensive alloying elements that are subject to great fluctuations with regard to their procurement costs. The steel and the flat steel product have for this purpose the following composition according to the invention (in % by weight): C: 0.12-0.18%; Si: 0.05-0.2%; Mn: 1.9-2.2%; Al: 0.2-0.5%; Cr: 0.05-0.2%; Nb: 0.01-0.06%; the remainder Fe and impurities that are unavoidable for production-related reasons, which include contents of phosphorus, sulfur, nitrogen, molybdenum, boron, titanium, nickel and copper as long as the following respectively apply for their contents: P: ≦0.02%, S: ≦0.003%, N: ≦0.008%, Mo: ≦0.1%, B: ≦0.0007%, Ti: ≦0.01%, Ni: ≦0.1%, Cu: ≦0.1%. Similarly, the invention relates to a method for producing a flat steel product that consists of a steel according to the invention.

The invention relates to a relatively high-strength steel that can be produced at low cost. Similarly, the invention relates to a flat steel product produced from such a steel and to a method for producing such a flat steel product.

When reference is made here to flat steel products, this refers to steel strips obtained by rolling processes, steel sheets and sheet bars, blanks and the like obtained therefrom.

Wherever figures are given here for the content of an alloying element in connection with an alloying specification, unless otherwise expressly stated they relate to the weight.

Dual-phase steels have already been used for some time in automobile construction. There are in this respect a large number of alloying concepts that are known for such steels, respectively composed to meet a wide variety of requirements. Many of the known concepts are based on alloying with molybdenum or presuppose elaborate production processes, in particular very rapid cooling down in the case of cold strip annealing, in order to produce the respectively desired microstructure of the steel. Since the price of molybdenum on the market is subject to strong fluctuations, the production of steels that contain high proportions of Mo entails a high cost risk. This is contrasted by the positive effects that molybdenum has with respect to the mechanical properties of dual-phase steels. For instance, sufficiently high Mo contents delay the formation of pearlite during cooling down, and thus ensure the creation of a microstructure that is favorable for the requirements imposed on the respective steel.

JP 11-310852 discloses a method for producing a hot strip from a dual-phase steel which contains (in % by weight) 0.03-0.15% C, up to 1.5% Si, 0.05-2.5% Mn, up to 0.05% P, 0.005-0.5% Al, 0.02-2% Cr, up to 0.01% N, up to 0.03% Ti, up to 0.06% Nb and, as the remainder, iron and unavoidable impurities. In this case, the contents of Mn and Cr should satisfy the condition that Cr+Mn≦3.5 and the contents of Ti and Nb should satisfy the condition that 0.005%≦2×Ti+Nb≦0.06%. The hot strip should in this case have a microstructure that consists (in % by unit area) of 55-95% polygonal ferrite and 5-45% hard phases that are formed at low temperatures. In order to achieve this, a correspondingly composed steel is cast into slabs, which after cooling down are heated up to 1280° C. and subsequently hot-rolled at a hot-rolling temperature of Ar3±50° C. to form hot strip. The hot strip obtained is then coiled at a coiling temperature of up to 250° C. The low coiling temperature leads to the formation of the strength-increasing phases, and thus to a very strong hot strip. However, this can only be further processed with difficulty. This is found in particular in the attempt to produce cold-rolled steel strip from hot strips produced in this way.

WO 2011/135997 likewise discloses a dual-phase steel, a hot-rolled steel strip produced therefrom, and a method for producing such a hot-rolled steel strip. Along with iron and unavoidable impurities, the steel consists here of (in % by weight) 0.07-0.2% C, 0.3-1.5% Si and Al, 1.0-3.0% Mn, up to 0.02% P, up to 0.005% S, 0.1-0.5% Cr and 0.001-0.008% N and also additionally 0.002-0.05% Ti or 0.002-0.05% Nb. The hot-rolled steel sheet has in this case a microstructure that consists (in % by unit area) of 7-35% ferrite with a particle diameter of 0.5-3.0 μm and, as the remainder, bainite-ferrite or bainite and martensite. High contents of at least 0.5% Si contribute in this case to increasing the strength of the steel, while aluminum is merely added to kill the steel during its production. Here, too, a low coiling temperature of less than 430° C. is prescribed, in order to ensure the formation of a sufficient amount of strength-increasing hard phases in the hot strip. Here, too, the setting of the microstructure already in the hot strip has the consequence that it is only with difficulty that the hot strip produced in this known way can be further processed into cold-rolled steel strip.

WO 2011/076383 also describes a hot-dip galvanized steel strip that is intended to have a high strength. The steel strip consists in this case of a steel that contains along with iron and unavoidable impurities (in % by weight) 0.10-0.18% C, 1.90-2.50% Mn, 0.30-0.50% Si, 0.50-0.70% Al, 0.10-0.50% Cr, 0.001-0.10% P, 0.01-0.05% Nb, up to 0.004% Ca, up to 0.05% S, up to 0.007% N, and optionally at least one of the following elements: 0.005-0.50% Ti, 0.005-0.50% V, 0.005-0.50% Mo, 0.005-0.50% Ni, 0.005-0.50% Cu and up to 0.005% B. The following applies here for the contents of Al and Si: 0.80%<Al+Si<1.05%, and for the contents of Mn and Cr: Mn+Cr>2.10%. The steel composed in this way is intended to offer improved deformability along with high strength and at the same time have good weldability and surface quality together with good producibility and coatability.

Against the background of the prior art explained above, the object of the invention was to provide a steel and a flat steel product that have optimized mechanical properties and at the same time can be produced at low cost, without having to rely for this on expensive alloying elements that are subject to great fluctuations with regard to their procurement costs.

In addition, a method that allows the reliable production of cold-rolled flat steel products of the kind that are to be produced according to the invention was to be provided.

According to the invention, this object has been achieved with respect to the steel by such a steel having the composition that is specified in claim 1.

With respect to the flat steel product, the solution according to the invention that achieves the aforementioned object is that such a flat steel product is constituted in the cold-rolled state as specified in claim 4.

With respect to the method, the aforementioned object has finally been achieved according to the invention by the working steps that are specified in claim 7 being implemented in the production of a cold-rolled flat steel product.

Carbon makes it possible for martensite to form in the microstructure, and is therefore an essential element for setting the desired high strength in the steel according to the invention. In order that this effect occurs to a sufficient extent, the steel according to the invention contains at least 0.12% by weight C. However, too high a C content has a negative effect on the welding characteristics. It generally applies here that the weldability of a steel decreases with the level of its carbon content. In order to avoid negative influences of the C content on its processability, in the case of the steel according to the invention the maximum carbon content is restricted to 0.18% by weight.

Silicon is likewise used for increasing strength, in that it increases the hardness of the ferrite. The minimum content of silicon of a steel according to the invention is for this purpose 0.05% by weight. However, too high a content of silicon leads both to the undesired grain boundary oxidation, which negatively influences the surface of a flat steel product produced from steel according to the invention, and to difficulties if a flat steel product according to the invention is to be hot-dip coated with a metallic coating to improve its corrosion resistance. In order to avoid such negative influences of Si in the steel according to the invention that make further processing more difficult, the upper limit of the Si content of a steel according to the invention is 0.2% by weight.

Manganese prevents the formation of pearlite during cooling down. As a result, in the steel according to the invention the desired martensite formation is promoted and the strength of the steel is increased. A sufficiently high content of manganese for suppressing pearlite formation lies here at 1.9% by weight. However, manganese also has the negative characteristic of forming segregations and of reducing the suitability for welding. In addition, the presence of relatively high Mn contents causes an increased expenditure of energy in the making of a steel according to the invention. In order to avoid the negative effects of Mn in the steel according to the invention, the upper limit of the content range envisaged for Mn of a steel according to the invention is 2.2% by weight.

Aluminum is of special significance in the alloy according to the invention. Even when contained in small amounts, it serves for deoxidation. The amount envisaged according to the invention of at least 0.2% by weight promotes the formation of residual austenite.

In a way similar to in known TRIP steels, this has a positive effect on the elongation after fracture and the n value of flat steel products produced from steel according to the invention. However, in the case where the steel according to the invention is cast into slabs or thin slabs as a primary product, an aluminum content of over 0.5% by weight impairs the properties of the slab and possibly leads to crack formation. Moreover, high contents of aluminum in the steel have negative effects on the coating characteristics. Therefore, in the case of a steel according to the invention the contents of Al are limited to 0.5% by weight.

Like manganese, chromium is present in the steel according to the invention to increase the strength. The presence of Cr has the effect of increasing the hardenability, and consequently the proportion of martensite in the steel. The Cr content required for this is at least 0.05% by weight. In order not to overdo the strength-increasing influence of Cr, at the same time the Cr content of a steel according to the invention is restricted to a maximum of 0.2% by weight.

Niobium forms ultrafine segregations in the steel according to the invention, and thereby likewise increases the strength. An Nb content of at least 0.01% by weight is required for this. Excessive contents would increase the positive influence of Nb too much and negatively influence the elongation after fracture. Therefore, in the case of a steel according to the invention, the Nb content is restricted to 0.06% by weight, the effect of Nb occurring with particular certainty if the Nb content is 0.01-0.04% by weight.

The amounts of any phosphorous, sulfur, nitrogen, molybdenum, boron, titanium, nickel and copper that may be contained in the steel according to the invention as impurities are so small that they have no influence on the properties of the steel and a flat steel product according to the invention produced therefrom. Accordingly, in a steel according to the invention, at most 0.02% by weight P, at most 0.003% by weight S, at most 0.008% by weight N, at most 0.1% by weight Mo, at most 0.0007% by weight B, at most 0.01% by weight Ti, at most 0.1% by weight Ni and at most 0.1% by weight Cu are respectively present, the content of molybdenum preferably lying below 0.05% by weight. It goes without saying that further impurities may be present in the steel according to the invention, getting into the steel for production-related reasons, for example due to the use of scrap. However, these impurities are likewise present in such small amounts in each case that they do not influence the properties of the steel. The sum of the contents of the alloying elements C, Si, Mn, Al, Cr and Nb present in effective amounts should be at least 2.5% by weight and should not exceed 3.5% by weight. If the sum of the alloying contents is too small, there is the risk that the desired mechanical properties are not achieved. If, on the other hand, the sum of the alloying contents is too high, a very high strength that is not desired here, of over 900 MPa, is achieved, together with poorer deforming characteristics.

The method according to the invention for producing a flat steel product according to the invention comprises the following working steps:

-   a) casting a steel composed according to the invention to form a     primary product, it being possible for the primary product to be a     slab or a thin slab;

b) hot rolling the primary product to form a hot strip with a thickness of 2 to 5.5 mm, the initial hot-rolling temperature being 1000-1300° C., in particular 1050-1200° C., and the final hot-rolling temperature being 840-950° C., in particular 890-950° C.;

-   c) coiling the hot strip to form a coil at a coiling temperature of     480-610° C.; -   d) cold rolling the hot strip to form a cold-rolled flat steel     product 0.6-2.4 mm thick, the degree of cold rolling achieved by     means of the cold rolling being 40-80%; -   e) annealing the cold-rolled flat steel product while it     continuously passes through, -   e.1) the cold-rolled flat steel product initially being heated in a     preheating stage at a heating-up rate of 0.2-45° C./s to a     preheating temperature of up to 870° C., -   e.2) the cold-rolled flat steel product subsequently being held at     an annealing temperature of 750-870° C. over an annealing period of     8-260 s in a holding stage, the preheated flat steel product     optionally being finish-heated to the respective annealing     temperature within this holding stage, -   e.3) the cold-rolled flat steel product being cooled down after the     end of the annealing period at a cooling-down rate of 0.5-110 K/s.

In order to be brought to the respectively required initial hot-rolling temperature before the hot finish-rolling, the respective primary product may if required stay in a furnace at a sufficient furnace temperature over a period of up to 500 minutes. Alternatively, the respective primary product may also be passed on to the hot rolling in the still sufficiently hot state.

The coiling temperature is fixed according to the invention at 480-610° C., because a lower coiling temperature would lead to a much stronger hot-rolled flat steel product (“hot strip”), which could only be further processed under more difficult conditions. A coiling temperature above 610° C., on the other hand, in combination with the chromium content envisaged according to the invention would increase the risk of grain boundary oxidation.

The coiled hot strip cools down in the coil to room temperature. Optionally, after cooling down it may be pickled, in order to remove scale and contaminants adhering to it.

After the coiling and pickling carried out if required, the hot strip is rolled in one or more cold rolling steps to form a cold-rolled flat steel product (“cold strip”). Starting from the thickness of the hot strip prescribed according to the invention, cold rolling is in this case performed with a total degree of cold rolling of 40-80%, in order to achieve the desired cold strip thickness of 0.6-2.4 mm.

In the next production step, the cold strip is subjected to continuous annealing. This serves firstly for setting the desired mechanical properties.

At the same time, it may be used for preparing the cold-rolled flat steel product for subsequent coating with a metallic coating, which protects the cold-rolled flat steel product from corrosive attacks during later use. On an industrial scale, such a coating can be applied in a particularly low-cost manner by hot-dip coating. The annealing envisaged according to the invention may in this case be carried out in a conventionally formed hot-dip coating installation of a continuous type. Alternatively, the annealing may also be followed by electrolytic galvanizing.

In the course of the heat treatment, both the heating up to the respective maximum annealing temperature and the subsequent cooling down may take place in one or more steps. The heating up takes place in this case initially in a preheating stage at a rate of 0.2 K/s to 45 K/s to a preheating temperature of up to 870° C., in particular 690-860° C. or 690-840° C.

Subsequently, the flat steel product runs into a holding stage, in which it reaches the maximum annealing temperature of 750-870° C. by undergoing further heating if its preheating temperature is less than the maximum annealing temperature respectively aimed for. The flat steel product is held at the respective maximum annealing temperature until the end of the holding stage is reached. The annealing period, within which the flat steel product is held respectively at the maximum annealing temperature in the holding stage, is 8-260 s. At too low a temperature or with too little time, the material would not recrystallize. As a consequence, on the one hand there would not be sufficient austenite available for the martensite formation for the microstructural transformation during the cooling. On the other hand, unrecrystallized steel would have the consequence of a definite anisotropy. By contrast, too long an annealing period or too high a temperature leads to a very coarse microstructure, and consequently to poor mechanical properties.

After completion of the annealing period, the cooling of the cold-rolled flat steel product takes place at a cooling-down rate of 0.5-110 K/s. The cooling-down rate is in this case set within this window in such a way that pearlite formation is avoided to the greatest extent.

If the cold-rolled flat steel product is intended to be hot-dip coated after the annealing, in the course of the cooling it is cooled down to a temperature of 455-550° C. The cold-rolled flat steel product adjusted in temperature in this way then runs through a molten Zn bath, which has a temperature of 450-480° C. If the temperature of the cold-rolled flat steel product falls into the range intended for the zinc bath, the steel strip can be held for a period of up to 100 s before entering the zinc bath. If, on the other hand, the temperature of the steel strip is greater than 480° C., up until the time it enters the zinc bath the flat steel product is cooled down at a cooling-down rate of up to 10 K/s, until its temperature falls within the temperature range intended for the zinc bath, in particular is equal to the temperature of the zinc bath.

On leaving the Zn bath, the thickness of the Zn-based protective layer present on the flat steel product is set in a known way by a stripping device.

Optionally, the hot-dip coating may be followed by a further heat treatment (“galvannealing”), in which the hot-dip coated flat steel product is heated to up to 550° C., in order to burn in the zinc layer.

Either directly after leaving the zinc bath or following the additional heat treatment, the cold-rolled flat steel product obtained is cooled down to room temperature.

The method according to the invention for producing flat steel products according to the invention consequently comprises the following variants:

Variant a)

The cold-rolled flat steel product (“cold strip”) is heated in a preheating furnace at a heating-up rate of 10-45 K/s to a preheating temperature of 660-840° C.

Subsequently, the preheated cold strip is passed through a furnace zone in which the cold strip is held at a temperature of 760-860° C. over a holding time of 8-24 s. Depending on the preheating temperature reached in the preceding working step, this causes further heating at a heating-up rate of 0.2-15 K/s.

The cold strip annealed in this way is then cooled down at a cooling-down rate of 2.0-30 K/s to an entry temperature of 455-550° C., with which it subsequently runs through a molten zinc bath and is held for a holding time of at most 45 s. The molten zinc bath has in this case a temperature of 455-465° C. Depending on its entry temperature, the cold strip cools down in the molten zinc bath at a cooling-down rate of up to 10 K/s to the respective temperature of the molten zinc bath or is held at a constant temperature. On the cold strip leaving the molten zinc bath, which is then provided with a zinc coating, the thickness of the coating is set in a way known per se. Finally, the coated cold strip is cooled to room temperature.

Variant b)

In an input heating zone of a continuous furnace, the cold-rolled flat steel product is brought to a target temperature, which is 760-860° C., at a heating-up rate of up to 25 K/s.

This is followed by holding of the thus heated-up cold-rolled flat steel product at an annealing temperature of 750-870° C., in particular 780-870° C., in a holding zone of the furnace for 35-150 s. Depending on the temperature at which the cold-rolled flat steel product enters the holding zone, it is thereby heated to the respective annealing temperature at a heating-up rate of up to 3 K/s during the holding time, i.e. within this holding zone.

The holding at the annealing temperature is followed by a two-stage cooling, in which the cold-rolled flat steel product is initially cooled down slowly at a cooling-down rate of 0.5-10 K/s to an intermediate temperature, which is 640-730° C., and is subsequently cooled down at an accelerated cooling-down rate of 5-110 K/s to a temperature of 455-550° C.

The cold-rolled flat steel product cooled down to the respective temperature then runs through a molten zinc bath. The molten zinc bath has in this case a temperature of 450-480° C. On the cold-rolled flat steel product leaving the molten zinc bath, which is then provided with a zinc coating, the thickness of the coating is set in a way known per se.

Following the application of the zinc coating, an annealing treatment (“galvannealing”) may be carried out, in order to bring about an alloy formation in the zinc coating. For this purpose, the cold strip provided with the zinc coating may be heated up to 470-550° C. and held at this temperature over a sufficient time.

After the zinc coating or, if such a treatment is carried out, after the galvannealing treatment, the zinc-coated cold strip may be subjected to a temper-rolling, in order to improve its mechanical properties and the surface condition of the coating. The degrees of tempering thereby set typically lie in the range of 0.1-2.0%, in particular 0.1-1.0%.

For setting its mechanical properties, the cold-rolled flat steel product composed and produced according to the invention may as an alternative to the possibility described above of hot-dip coating also run through a heat treatment in a conventional annealing furnace, in which the heating up (working step e.1)) and the annealing at a respective annealing temperature (working step e.2) are performed in the way described above, in which however the working step e.3 is carried out at least in two stages, in that the cold-rolled flat steel product is initially cooled down to a temperature range of 250-500° C., then stays in this temperature range for up to 760 s and is subsequently cooled down further. In this way, the residual austenite in the microstructure of the flat steel product according to the invention is stabilized.

In the case of a variant of the method according to the invention within this procedure, the following heat treatment steps are then run through in a continuous furnace:

The cold-rolled flat steel product is first heated up at a heating-up rate of 1-8 K/s to 750-870, in particular 750-850° C., in a heating zone.

Subsequently, the thus heated cold-rolled flat steel product is passed through a furnace zone in which the cold-rolled flat steel product is held at an annealing temperature of 750-870° C., in particular 750-850° C., over a holding time of 70-260 s. Depending on the preheating temperature reached in the preceding working step, this involves further heating up at a heating-up rate of up to 5 K/s.

The thus annealed cold-rolled flat steel product is subsequently subjected to a two-stage cooling, in which it is cooled down initially at an accelerated cooling-down rate of 3-30 K/s to an intermediate temperature of 450-570° C. This cooling can then be performed as air and/or gas cooling. This is followed by slower cooling, in which the cold-rolled flat steel product is cooled down at a cooling-down rate of 1-15 K/s to 400-500° C.

The respective cooling may be followed by an overaging treatment, in which the cold-rolled flat steel product is held at a temperature of 250-500° C., in particular 250-330° C., over a holding time of 150-760 s. Depending on the respective entry temperature, this involves cooling of the cold-rolled flat steel product at a cooling-down rate of up to 1.5 K/s.

The cold-rolled flat steel product heat-treated in the way described above may finally be subjected to a temper-rolling, in order to improve its mechanical properties further. Here, too, the degrees of tempering thereby set typically lie in the range of 0.1-2.0%, in particular 0.1-1.0%.

The thus heat-treated, and possibly temper-rolled, cold-rolled flat steel product may subsequently run through a coating installation for electrolytic coating, in which the respective metallic protective layer, for example a zinc alloy layer, is electrochemically (“electrolytically”) deposited in a way known per se on the cold-rolled flat steel product.

A flat steel product according to the invention has an alloy according to the invention that is composed in the way explained above and is moreover characterized by a microstructure that consists of 50-90% by volume ferrite, including bainitic ferrite, 5-40% by volume martensite, up to 15% by volume residual austenite and up to 10% by volume other structural constituents that are unavoidable for production-related reasons, the residual austenite content optimally lying in the range of 6-12% by volume.

The characteristic values determined in the tensile test according to DIN EN ISO 6892 (specimen form 2, longitudinal specimens) thereby lie in the following ranges:

R_(p0.2) at least 440 MPa, in particular up to 550 MPa, R_(m) at least 780 MPa, in particular up to 900 MPa, A₈₀ at least 14%, n_(10-20/Ag) at least 0.10, BH₂ at least 25 MPa, in particular at least 30 MPa.

In practice, flat steel products according to the invention can be reliably produced by using the method according to the invention.

Respectively represented in the diagrams reproduced in FIGS. 1 and 2 are different temperature profiles that occur when the cold-rolled flat steel product runs through an annealing performed in the way according to the invention with directly following hot-dip coating:

-   -   preheating to a preheating temperature TV by means of a         heating-up rate RV;     -   holding at a maximum annealing temperature TG over an annealing         period tG, the holding comprising a finish-heating to the         annealing temperature TG if the preheating temperature TV is         lower than the annealing temperature TG (dashed line TV=TG;         solid line TV<TG);     -   cooling down in one stage (FIG. 1) or two stages (FIG. 2) as         follows:         -   cooling down of the flat steel product to a temperature TE             (FIG. 1) or TE′ (FIG. 2),         -   optional holding at the temperature TE over a period tH if             the respective temperature TE falls within the temperature             range intended for the temperature TB of the molten bath, in             particular is equal to the temperature TB, (FIG. 1)     -   or         -   further cooling down, starting from the temperature TE′, to             a temperature TE″ if the temperature TE′ is greater than the             upper limit of the temperature range intended for the molten             bath, the temperature TE″ reached in the second cooling step             falling within the temperature range intended for the             temperature TB of the molten bath, in particular being equal             to the temperature TB, (FIG. 2);     -   passing the flat steel product through a molten bath within a         running-through time tB;     -   cooling down to room temperature RT.

On the other hand, indicated by way of example in the diagram according to FIG. 3 is a temperature profile that occurs if the flat steel product runs through a continuous annealing without subsequent hot-dip coating:

-   -   preheating to a preheating temperature TV within a preheating         period tV at a heating-up rate RV;     -   holding at a maximum annealing temperature TG over an annealing         period tG, the holding comprising a finish-heating to the         annealing temperature TG if the preheating temperature TV is         lower than the annealing temperature TG (dashed line TV=TG;         solid line TV<TG);     -   cooling down in two stages, being cooled down in the first stage         at a higher cooling-down rate to an intermediate temperature TZ′         and subsequently at a reduced cooling-down rate to an         intermediate temperature TZ″ and the cooling lasting altogether         for a cooling-down period of tZ;     -   carrying out an overaging treatment, in which the flat steel         product is cooled down to an overaging temperature TU from the         intermediate temperature TZ″ at a cooling-down rate RU over a         treatment period tU;     -   cooling down to room temperature RT.

For checking the effects achieved by the invention, nine steel melts A-I, the compositions of which are given in Table 1, were melted. The steels A-H are steels according to the invention, while the steel I is outside the invention.

The steel melts A-I were cast into slabs and, after cooling, heated in a furnace to the respective initial hot-rolling temperature WAT.

In the course of the hot rolling, the slabs running into the group of hot-rolling stands with the initial hot-rolling temperature WAT were hot-rolled at a final temperature WET to form hot-rolled steel strips with a thickness WBD. After the hot rolling, the hot-rolled steel strips were cooled down to a coiling temperature HT, at which they were subsequently wound into a coil and cooled down to room temperature.

The hot-rolled steel strips thus obtained were cold-rolled with a respective overall degree of deformation KWG to form cold-rolled steel strip with a thickness KBD.

The operating parameters taken into consideration in the production of the hot- and cold-rolled steel strips, the “initial hot-rolling temperature WAT”, the “final hot-rolling temperature WET”, the “thickness of the hot-rolled steel strip WBD”, the “coiling temperature HT”, the “overall degree of deformation KWG” and the “thickness of the cold-rolled steel strip KBD”, are given in Tables 2 and 3.

The cold-rolled steel strips thus obtained were subjected to different annealing tests.

In the case of the first variant of these tests, following the profile represented in FIG. 1, in a conventional hot-dip coating installation steel strips were initially heated up to a preheating temperature TV in a preheating zone at a heating-up rate RV.

Directly following the preheating, the steel strips were initially finish-heated at a heating-up rate RF in a holding zone up to a maximum annealing temperature TG, at which they were subsequently held. For running through the entire holding zone, i.e. including the finish-heating and the holding, an annealing period tG was required.

Following similarly without interruption, the cold-rolled steel strips were then cooled down to a temperature TE in one stage at a cooling-down rate RE. The steel strips leaving the molten bath had a Zn-alloy coating, which protects them from corrosion.

The operating parameters taken into consideration in the production of the hot- and cold-rolled steel strips, the “heating-up rate RV”, the “preheating temperature TV”, the “heating-up rate RF”, the “annealing temperature TG”, the “annealing period tG”, the “cooling-down rate rE”, the “temperature TE”, the “holding time tE”, the “cooling-down rate RB” and the “bath temperature TB”, are given in Table 4.

In the case of the second variant of these tests, following the profile represented in FIG. 2, in a conventional hot-dip coating installation steel strips were in turn initially heated up to a preheating temperature TV in a preheating zone at a heating-up rate RV. Directly following the preheating, the steel strips ran into a second zone of the respective furnace. If their preheating temperature TV was less than the prescribed maximum annealing temperature TG, the steel strips were finish-heated at a heating-up rate RF up to the required maximum annealing temperature TG. The steel strips heated up to the respective annealing temperature TG were then held at this temperature over an annealing period tG. Following without interruption, the cold-rolled steel strips were then cooled down in two stages. In the first stage of the cooling, the steel strips were cooled down to an intermediate temperature TE′ at a comparably low cooling-down rate RE′. On reaching the intermediate temperature TE′, the respective steel strips were quickly cooled down to the respective temperature TE at an increased cooling-down rate RE. The steel strips leaving the molten bath had a Zn-alloy coating, which protects them from corrosion.

The operating parameters taken into consideration in the production of the hot- and cold-rolled steel strips, the “heating-up rate RV”, the “preheating temperature TV”, the “heating-up rate RF”, the “annealing temperature TG”, the “annealing period tG”, the “cooling-down rate RE′”, the “intermediate temperature TE′”, the “cooling-down rate RE”, the “temperature TE”, the “holding time tE”, the “cooling-down rate RB” and the “temperature TB”, are given in Table 5.

In the case of the third variant of the tests, following the profile represented in FIG. 3, in a conventional heat-treatment installation steel strips were initially heated up to a preheating temperature TV in a preheating zone at a heating-up rate RV. Directly following the preheating, the steel strips ran into a second zone of the respective furnace. If their preheating temperature TV was less than the prescribed maximum annealing temperature TG, the steel strips were finish-heated in this holding zone at a heating-up rate RF up to the required maximum annealing temperature TG. The steel strips heated up to the respective annealing temperature TG were then held at this temperature. The finish-heating and the holding thereby likewise took place altogether in an annealing period tG.

Following without interruption, the cold-rolled steel strips were then cooled down in two stages. In the first stage of the cooling, the steel strips were cooled down to an intermediate temperature TZ′ at a comparably high cooling-down rate RZ′ by use of gas-jet cooling. On reaching the intermediate temperature TZ′, the gas-jet cooling was ended and roller cooling took place at a reduced cooling-down rate RZ″ down to an intermediate temperature TZ″. The two-stage cooling was followed by an overaging treatment, by way of which the respective steel strip was cooled down from the intermediate temperature TZ″ to the overaging temperature TU at a cooling-down rate RU.

The operating parameters taken into consideration in the production of the hot- and cold-rolled steel strips, the “heating-up rate RV”, the “preheating temperature TV”, the “heating-up rate RG”, the “annealing temperature TG”, the “annealing period tG”, the “cooling-down rate RZ′”, the “intermediate temperature TZ′”, the “cooling-down rate RZ″”, the “intermediate temperature TZ″”, the “cooling-down rate RU” and the “overaging temperature TU”, are given in Table 6.

Each of the cold-rolled steel sheets obtained by the tests described above was in each case finally temper-rolled with a degree of temper-rolling DG. This applies both to the steel strips that were hot-dip coated in the first two series of tests and also to the steel strips that ran through the third series of tests.

On the cold-rolled steel strips produced in the way described above, the yield strength Rp0.2, the tensile strength Rm, the elongation A80, the n value (10-20/Ag) and the composition of the microstructure were determined, these properties respectively being determined on specimens longitudinally in relation to the rolling direction.

In addition, the V-bending behavior in accordance with DIN EN ISO 7438 was determined. The ratio of the minimum bending radius, that is to say the radius at which no visible crack occurs, to the sheet thickness should be at most 1.5 here, and ideally should not exceed 1.0.

Similarly, in the bending test in accordance with DIN EN ISO 7438 (specimen dimensions sheet thickness*20 mm*120 mm), the minimum bending dome diameter at which no visible damage occurs was determined. It should be 2*sheet thickness, ideally 1.5*sheet thickness. With respect to the present invention, this means that the maximum bending dome diameter should not exceed 4.8 mm.

Finally, on punched specimens of the cold-rolled steel strips produced in the way described above, the hole expansion was determined in accordance with ISO 16630, with a hole diameter of 10 mm at a drawing rate of 0.8 mm/s. It is at least 14%, ideally at least 16%.

In Table 7 it is indicated for the altogether 58 tests carried out in the way described above which of the steels indicated in Table 1 was processed, which of the hot-rolling variants indicated in Table 2 was applied, which of the cold-rolling variants indicated in Table 3 was used and which of the annealing method variants respectively indicated in Tables 4, 5 and 6 was run through by the respective cold-rolled steel strip. Furthermore, the respective degree of tempering DG, the mechanical properties and the composition of the microstructure as well as the properties determined in accordance with DIN EN ISO 7438 (“V-bend”, “U-bend”) and DIN ISO 16630 (“hole expansion”) are indicated in Table 7.

TABLE 1 Steel C Si Mn P S Al Cr Nb Mo N B Ti Ni Cu Total A 0.162 0.117 1.97 0.013 0.0005 0.379 0.115 0.020 0.003 0.0031 0.0003 0.005 0.019 0.018 2.82 B 0.154 0.112 1.96 0.012 0.0009 0.380 0.112 0.017 0.019 0.0076 0.0002 0.009 0.069 0.024 2.88 C 0.124 0.194 2.05 0.011 0.0007 0.353 0.090 0.034 0.009 0.0029 0.0005 0.007 0.024 0.005 2.91 D 0.171 0.052 1.91 0.018 0.0026 0.340 0.153 0.027 0.003 0.0060 0.0003 0.003 0.032 0.030 2.75 E 0.143 0.085 2.18 0.008 0.0010 0.272 0.051 0.012 0.094 0.0043 0.0003 0.002 0.044 0.049 2.95 F 0.169 0.135 1.95 0.017 0.0028 0.210 0.179 0.029 0.025 0.0037 0.0004 0.004 0.026 0.017 2.77 G 0.174 0.097 1.94 0.010 0.0022 0.481 0.113 0.024 0.038 0.0027 0.0002 0.006 0.014 0.009 2.91 H 0.152 0.135 2.11 0.006 0.0019 0.371 0.194 0.039 0.050 0.0031 0.0006 0.005 0.018 0.052 3.14 I 0.164 0.120 1.91 0.014 0.0010 0.067 0.110 0.021 0.004 0.0026 0.0005 0.004 0.017 0.038 2.47 (all figures are given in % by weight, the remainder iron and unavoidable impurities)

TABLE 2 Hot rolling WAT WET HT [° C.] [° C.] [° C.] I 1050 920 550 II 1200 920 550 III 1150 880 550 IV 1150 950 580 V 1150 900 490 VI 1150 920 610 VII 1150 920 550

TABLE 3 Cold rolling WBD KWG KBD [mm] [%] [mm] a 2.29 65 0.8 b 2.86 65 1.0 c 5.00 80 1.0 d 4.44 55 2.0 e 5.00 60 2.0 f 4.00 40 2.4

TABLE 4 Heating zone Holding zone Gas-jet cooling (heating) (finish-heating holding) (1st cooling step) Zinc bath RV TV RF TG tG RE TE tE RB TB [° C./s] [° C.] [° C./s] [° C.] [s] [° C./s] [s] [° C.] [° C./s] [° C.] 1.1 18.7 795 1 860 17.2 5.1 505 1.6 460 1.2 18.3 780 1.2 855 17.2 5.1 500 1.6 455 1.3 22.9 720 1.9 810 12.3 6.6 475 0.6 465 1.4 20.9 800 0.6 835 15.5 5.1 515 2.3 455 1.5 10.4 700 0.9 790 23.6 3.0 460 39.6 0 460 1.6 34.6 740 2.8 820 8.6 8.9 520 4.5 455 1.7 38.2 760 3.7 860 8.0 13.6 460 13.4 0 460

TABLE 5 Heating zone Holding zone Slow cooling Rapid cooling (heating) (finish-heating holding) (1st cooling step) (2nd cooling step) Zinc bath RV TV RF TG tG RE′ TE′ RE TE tE″ RB TB [° C./s] [° C.] [° C./s] [° C.] [s] [° C./s] [° C.] [° C./s] [° C.] [s] [° C./s] [° C.] 2.1 7.2 760 0.2 780 1 640 21.1 460 96 460 2.2 23.9 860 0 860 39 5.9 730 85.9 510 2.3 455 2.3 6.4 800 0 800 147 2.4 680 40.1 460 69 460 2.4 11.1 800 0.2 820 3.4 650 25.5 510 1 460 2.5 12.7 860 0 860 74 4.1 680 46.9 455 45 455 2.6 15.9 780 1.4 830 7.6 650 15.2 520 0.7 465

TABLE 6 Heating zone Holding zone Gas-jet cooling Roller cooling (heating) (finish-heating holding) (1st cooling step) (2nd cooling step) Overaging RV TV RG TG tG RZ′ TZ′ RZ″ TZ″ RU TU [° C./s] [° C.] [° C./s] [° C.] [s] [° C./s] [° C.] [° C./s] [° C.] [° C./s] [° C.] 3.1 2.3 750 0.7 850 — 10.9 550 3.3 470 0.4 290 3.2 2.3 810 810 170 10.1 500 1.1 470 0.4 260 3.3 2.9 830 830 140 11.7 560 4.5 470 0.4 320 3.4 1.7 780 780 220 5.8 550 2.3 470 0.3 290 3.5 4.3 810 0.2 830 26.2 450 2.4 420 0.5 290 3.6 5.3 850 850 75 21.7 550 4.2 500 0.9 290

TABLE 7 Microstructure [% by volume] Hole Hot Cold Anneal- DG R_(p0.2) R_(m) A80 n Fer- Martens- Remaining V-bend U-bend expan- Steel rolling rolling ing [%] [MPa] [MPa] [%] value rite ite austenite Other [minR1/d] [D1] sion 1 A I a 1.7 0.6 453 824 18.6 0.131 70 20 9.5 0.5 0.63 0.8 15 2 A I b 1.1 0.6 475 843 16.7 0.113 60 30 6.5 3.5 1.50 1 16 3 A I d 1.1 0.6 486 860 17.6 0.133 50 40 8 2 0.75 4 18 4 A II e 1.2 0.2 442 784 20.4 0.148 80 5 12.5 2.5 0.50 4 25 5 B II e 1.2 0.6 464 820 19.3 0.151 70 25 5 0 0.25 2 14 6 B III c 1.5 0.3 444 812 20.9 0.152 70 15 11 4 0.50 2 16 7 C III f 1.4 0.6 485 829 20.2 0.180 60 30 9 1 0.63 2.4 15 8 C IV b 1.5 0.6 473 792 20.5 0.175 75 15 7 3 1.00 1 14 9 D IV a 1.3 0.6 456 782 19.8 0.137 76 10 6.5 8.6 1.25 1.6 17 10 D V b 1.3 0.6 481 809 17.8 0.131 70 20 8 2 1.00 1 19 11 E V c 1.5 0.6 443 834 18.8 0.132 65 25 9.5 0.5 1.50 2 19 12 E VI f 1.6 0.6 482 794 19.5 0.134 80 10 7.5 2.5 0.83 4.8 15 13 F VI a 1.8 0.6 452 812 17.0 0.116 75 10 10.6 4.5 0.63 0.8 21 14 F VI f 1.5 0.6 490 795 20.4 0.150 70 20 8.5 1.5 1.04 4.8 16 15 G VII d 1.3 0.6 454 817 21.4 0.151 65 20 10 5 0.75 4 19 16 G I e 1.7 0.3 441 813 23.4 0.156 65 15 13.5 6.5 0.60 2 23 17 H VII a 1.2 0.7 501 849 14.7 0.111 55 36 6.5 3.5 1.25 0.8 15 18 H VII f 1.2 1.0 535 813 17.0 0.116 80 10 8.5 1.5 0.63 4.8 17 19 A I b 2.1 0.6 495 843 16.7 0.113 60 30 6 4 0.50 1 15 20 A I d 2.2 0.6 449 818 19.4 0.138 70 20 7.5 2.5 1.25 4 20 21 A II e 2.4 0.2 480 817 20.3 0.142 76 15 5 5 1.50 2 14 22 B II e 2.2 0.6 449 798 20.3 0.157 75 10 14 1 1.00 4 18 23 B III c 2.5 0.3 459 787 19.1 0.134 70 15 10.5 4.5 1.50 2 20 24 C III f 2.4 0.6 496 784 20.8 0.192 75 10 15 0 0.63 2.4 24 25 C IV b 2.5 0.6 485 802 18.0 0.142 80 15 5 0 2.00 2 14 26 D IV a 2.3 0.6 466 796 19.6 0.135 80 10 8 2 1.25 1.6 19 27 D V b 2.3 0.6 450 816 18.1 0.113 76 15 9.5 0.5 1.00 1 16 28 E V c 2.5 0.6 479 842 16.7 0.115 60 35 6 0 1.50 2 15 29 E VI f 2.5 0.6 461 825 17.3 0.119 65 25 9 1 0.63 2.4 17 30 F VI a 2.6 0.6 509 804 19.2 0.146 65 20 13.5 1.5 0.63 0.8 16 31 F VI f 2.5 0.6 464 794 21.3 0.152 70 10 14.5 5.5 0.83 4.8 21 32 G VII d 2.3 0.6 495 857 17.4 0.117 50 40 6 4 1.25 4 15 33 G I e 2.6 0.3 447 832 19.9 0.133 70 20 8.5 1.5 1.00 4 19 34 H VII a 2.1 0.7 473 833 15.9 0.129 70 20 8 2 1.25 0.8 18 35 H VII f 2.2 0.9 490 808 17.5 0.120 75 15 7 3 1.04 4.8 19 36 A I b 3.1 0.6 481 821 21.2 0.150 70 10 14 6 1.00 1 20 37 A I d 3.2 0.6 538 844 20.5 0.148 65 26 8.5 1.5 0.75 4 19 38 A II e 3.4 0.2 445 805 20.1 0.146 75 10 9.5 5.5 0.50 4 18 39 B II e 3.2 0.6 441 785 20.4 0.123 85 8 6.5 3.5 0.50 2 16 40 B III c 3.5 0.3 524 825 18.4 0.121 70 20 5.6 4.5 1.00 2 15 41 C III f 3.4 0.6 469 813 18.4 0.126 66 20 10.5 4.5 1.25 2.4 16 42 C IV b 3.5 0.6 475 801 18.5 0.175 70 15 16 0 0.50 2 24 43 D IV a 3.3 0.6 497 806 20.3 0.136 75 10 10 6 0.63 1.6 20 44 D V b 3.3 0.6 497 789 22.1 0.148 70 10 13.5 6.5 0.60 1 22 45 E V c 3.6 0.6 454 824 17.8 0.115 75 15 7.5 2.5 1.50 1 15 46 E VI f 3.5 0.6 497 842 17.4 0.114 60 30 7 3 0.83 4.8 14 47 F VI a 3.6 0.6 506 832 17.8 0.116 65 25 8.6 1.6 1.25 0.8 16 48 F VI f 3.5 0.6 473 825 20.6 0.144 70 15 12.5 2.5 0.83 4.8 21 49 G VII d 3.3 0.6 450 837 19.1 0.125 65 25 9 1 1.00 4 17 50 G I e 3.6 0.3 461 830 18.8 0.130 66 25 8 2 1.25 4 19 51 H VII a 3.1 0.7 463 803 19.3 0.143 70 15 11 4 1.25 0.8 20 52 H VII f 3.2 0.9 445 818 17.3 0.122 70 20 5.5 4.6 0.00 2.4 17 53 I II a 3.1 0.6 438 776 21.6 0.159 80 4 5 17 1.25 1.6 14 54 I II a 2.1 0.6 427 715 15.8 0.098 90 5 4 1 1.88 3.2 11 55 I II a 2.2 0.6 507 740 16.0 0.125 75 5 5 15 1.25 2.4 14 56 I II a 1.1 0.6 517 858 16.6 0.101 50 42 4 4 1.88 3.2 12 57 I II a 1.2 0.6 439 750 16.1 0.108 85 5 3.6 6.5 1.25 1.6 14 58 I II a 3.2 0.6 439 760 16.6 0.108 90 3 4 3 1.88 3.22 10 

1. A steel comprising the following composition (in % by weight) C: 0.12 −0.18%; Si: 0.05 −0.2%; Mn: 1.9 −2.2%; Al: 0.2 −0.5%; Cr: 0.05 −0.2%; Nb: 0.01 −0.06%; the remainder Fe and impurities that are unavoidable for production-related reasons, which include contents of phosphorus, sulfur, nitrogen, molybdenum, boron, titanium, nickel and copper as long as the following respectively apply for their contents: P: ≦0.02% S: ≦0.003% N: ≦0.008% Mo: ≦0.1% B: ≦0.0007% Ti: ≦0.01% Ni: ≦0.1% Cu: ≦0.1%.
 2. The steel as claimed in claim 1, wherein its Mo content is at most 0.05% by weight.
 3. The steel as claimed in claim 1, wherein the sum of the contents of C, Si, Mn, Al, Cr and Nb is 2.5-3.5% by weight.
 4. A cold-rolled flat steel product, where the flat steel product has a composition as claimed in claim 1 and a microstructure that consists of 50-90% by volume ferrite, including bainitic ferrite, 5-40% by volume martensite, up to 15% by volume residual austenite and up to 10% by volume other structural constituents that are unavoidable for production-related reasons.
 5. The flat steel product as claimed in claim 4, wherein its content of residual austenite is 6-12% by volume.
 6. The flat steel product as claimed in claim 4, wherein its yield strength R_(p0.2) is at least 440 MPa, its tensile strength Rm is at least 780 MPa, its elongation after fracture A80 is at least 14%, its n_(10-20/Ag) is at least 0.1 and its BH2 value is at least 25 MPa.
 7. A method for producing a cold-rolled flat steel product constituted as claimed in claim 4, the method comprising the following working steps: a) casting a steel comprising the following composition (in % by weight) C: 0.12 −0.18% Si: 0.05 −0.2%; Mn: 1.9 −2.2%; Al: 0.2 −0.5% Cr: 0.05 −0.2%; Nb: 0.01 −0.06%; the remainder Fe and impurities that are unavoidable for production-related reasons, which include contents of phosphorus, sulfur, nitrogen, molybdenum, boron, titanium, nickel and copper as long as the following respectively apply for their contents: P: ≦0.02% S: ≦0.003% N: ≦S 0.008% Mo: ≦0.1% B: ≦0.0007% Ti: ≦0.01% Ni: ≦0.1% Cu: ≦0.1% composed to form a primary product; b) hot rolling the primary product to form a hot strip with a thickness of 2 to 5.5 mm, the initial hot-rolling temperature being 1000-1300° C. and the final hot-rolling temperature being 840-950° C.; c) coiling the hot strip to form a coil at a coiling temperature of 480-610° C.; d) cold rolling the hot strip to form a cold-rolled flat steel product 0.6-2.4 mm thick, the degree of cold rolling achieved by means of the cold rolling being 40-80%; e) continuous annealing the cold-rolled flat steel product, e.1) the cold-rolled flat steel product initially being heated in a preheating stage at a heating-up rate of 0.2-45° C./s to a preheating temperature of up to 870° C., e.2) the cold-rolled flat steel product subsequently being held at an annealing temperature of 750-870° C. over an annealing period of 8-260 s in a holding stage, the preheated flat steel product optionally being finish-heated to the respective annealing temperature within the holding stage, e.3) the cold-rolled flat steel product being cooled down after the end of the annealing period at a cooling-down rate of 0.5-110 K/s.
 8. The method as claimed in claim 7, wherein, before working step b), the primary product is heated to the respective initial hot-rolling temperature over a heating-up period of up to 500 minutes.
 9. The method as claimed in claim 7, wherein, after working step a), the primary product is cooled down to the respective initial hot-rolling temperature and passed on directly thereafter to the hot rolling.
 10. The method as claimed in claim 7, wherein the cold-rolled flat steel product passes through a hot-dip coating, which follows on in the continuous flow from working step e.3), and wherein the temperature to which the cold-rolled flat steel product is cooled down in working step e.3) is 455-550° C.
 11. The method as claimed in claim 7, wherein the cold-rolled flat steel product is cooled down to room temperature in working step e.3).
 12. The method as claimed in claim 11, wherein the cold-rolled flat steel product is cooled down in at least two cooling-down steps in working step e.3).
 13. The method as claimed in claim 11, wherein the cold-rolled flat steel product is cooled down to 250-500° C. in working step e.3) and is held in this temperature range for up to 760 s, in order to carry out an overaging treatment, and in that the cold-rolled flat steel product is subsequently finish-cooled.
 14. The method as claimed in claim 11, wherein, after the cooling down to room temperature, the cold-rolled flat steel product is electrolytically covered with a metallic protective coating.
 15. The method as claimed in claim 7, wherein the cold-rolled flat steel product is finally temper-rolled with a degree of tempering of 0.1-2.0%. 